This invention relates to a diaphragm excelling in chemical properties, particularly in alkali resisting property and to a method for the manufacture of said diaphragm. More particularly, the present invention relates to a novel diaphragm which substantially comprises a fluorine-containing polymer and potassium titanate, excels in hydrophilicity, possesses low electric resistance and exhibits good water permeability and offers high resistance to chemicals, particularly to alkalis and also to a method for the manufacture of said diaphragm.
Generally diaphragms of fluorine-containing polymers are used in diaphragm-type electrolytic cells for the decomposition such as of sodium chloride or water, fuel cells and alkali cells each for the purpose of keeping two kinds of gases or liquids from mingling with each other.
For use in the fields involving electrolytic processes and electric cell production, the diaphragms are required to possess certain properties. Examples of desirable properties are (1) excellent permeability to ions, (2) low electric resistance, (3) distribution of voids of a uniform, fine diameter, (4) excellent uniformity of current distribution, (5) electrical non-conductivity of the diaphragm itself (6) resistance to corrosion, (7) stability enough to withstand variations in conditions such as temperature and pH and (8) sufficient mechanical strength. Heretofore, fluorine-containing polymers have been known to be capable of withstanding the chemical actions such as of chlorine and alkalis and many methods have been proposed for the production of diaphragms from fluorine-containing polymers. Various products made of these polymers are now available on the market. Because of lack of hydrophilicity and a gradual rise in electric resistance experienced during their continued use, however, these products cannot be used in their unaltered form. If the pores in the diaphragm are given increased diameter for the purpose of lowering the diaphragm's electric resistance, then the volume of water permitted to pass through the diaphragm increases and the capacity of the diaphragm for separation of anolyte and catholyte is lowered possibly to the extent of completely depriving the diaphragm of its intended utility. It is, therefore, an important task for the industry to develop a diaphragm which excels in resistance to chemicals and possesses low electric resistance and proper water permeability both adequate for the uses to which the diaphragm is put.
For the accomplishment of this task, there have been suggested various methods for conferring hydrophilicity upon the diaphragms made of fluorine-containing polymers. A method which comprises immersing a diaphragm in an organic solvent such as, for example, methanol, acetone or methylethyl ketone which sufficiently wets fluorine-containing polymers and yet is soluble in water and immediately transferring the diaphragm in its swelled state into water (Japanese patent publication No. 38603/71, Japanese patent disclosure No. 52681/73) and a method which comprises coating the surface of a diaphragm of fluorine-containing polymer with a surface active agent are typical of the various methods (Japanese patent publication No. 38603/71, Japanese patent disclosure No. 1377/76). Since these agents which serve to confer hydrophilicity upon diaphragms are soluble in media with which the diaphragms are destined to remain in intimate contact, the diaphragms when used in electrolytic solutions containing dissolved gases or when used in electrochemical reactions involving evolution of gases, are liable to resume their inherent hydrophobic nature owing to the high affinity fluorine-containing polymers exhibit for gases. Besides, methods which produce diaphragms by mixing powdered fluorine-containing polymers with hydrophilic powders such as of silicon dioxide, titanium oxide, zirconium oxide and calcium fluoride or with hydrophilic cellulosic substances such as glass fiber, zirconia fiber and asbestos fiber and forming the resultant mixtures in required shapes (Japanese patent publication No. 25267/71, Japanese patent publication No. 978/73 and Japanese patent disclosure No. 26770/75 ) and a method which comprises mixing a powdered fluorine-containing polymer with a pore-forming agent capable of being dissolved out and forming the resultant mixture in a required shape (Japanese patent disclosure No. 81279/74) have also been known to the art. The diaphragms obtained by these methods excel those obtained by the aforementioned methods resorting to use of agents for conferring hydrophilicity in terms of the durability of acquired hydrophilicity. Nevertheless, they hardly can be regarded as possessing sufficient hydrophilicity. Although it may be suggested that improvement of the hydrophilicity may be attained by decreasing the particle diameter of the hydrophilic powder or even by increasing the amount thereof, a diaphragm obtained in consequence of such a measure would show extremely lowered strength. In the case of a diaphragm having polytetrafluoroethylene as its fluorine-containing polymer, the presence of such a hydrophilic powder causes the diaphragm to develop cracks when it is subjected to burning in the course of manufacture, making it extremely difficult to produce a product of satisfactory shape. There has also been known a method which comprises immersing a porous fluorine-containing polymer in a suspension having dispersed therein a finely divided hydrophilic powder such as of silicon dioxide or titanium oxide for thereby causing silicon dioxide or titanium oxide to be deposited inside the voids of the polymer (Japanese patent publication No. 38603/71). In the diaphragm obtained by this method, the hydrophilicity proves to be satisfactory. It is nevertheless extremely difficult to have solid particles uniformly deposited on the interior of diaphragm, particularly onto the inner surfaces of voids of very small diameters. This diaphragm has an additional disadvantage in that the finely divided particles deposited inside the diaphragm are readily released while the diaphragm is in use. These various disadvantageous phenomena point to a possibility that the hydrophilicity desired to be conferred upon a diaphragm throughout the entire thickness thereof without reference to the fineness of the diameter of voids present therein may be realized advantageously by a method which comprises allowing such solid particles to permeate in the form of a solution into the diaphragm and subsequently converting the deposited powdered substance into an insoluble compound for thereby causing the deposited particles to be educed uniformly within the diaphragm. Methods which rely upon this process for eduction of the hydrate of titanium oxide within a diaphragm have been suggested to the art (Japanese patent disclosure No. 81281/74 and Japanese patent disclosure No. 1373/76).
Undeniably, diaphragms obtained by these methods have excellent qualities in respect of hydrophilicity. In terms of resistance to chemicals, particularly to alkalis, however, they are not perfect. If such a diaphragm is used in an aqueous solution containing 30% (by weight; the same is applicable hereinafter) of sodium hydroxide or potassium hydroxide at an elevated temperature, for example, it undergoes a change of weight of such magnitude as to become no longer usable.
The inventors made studies in search of a stable diaphragm which possesses both hydrophilicity and resistance to chemicals, particularly to alkalis. They have consequently made a discovery that there can be manufactured a diaphragm capable of being advantageously used under conditions of a high temperature not lower than 90.degree. C. and an alkali concentration not less than 30%. The present invention has been accomplished on the basis of this discovery.
An object of the present invention is to provide a diaphragm which excels in hydrophilicity and resistance to chemicals.
Another object of this invention is to provide a method for the manufacture of a diaphragm excelling in hydrophilicity and resistance to chemicals.